The invention relates generally to ignition systems for internal combustion engines. More particularly, the invention relates to electronic ignition systems for marine propulsion devices.
Previously known electronic ignition systems for internal combustion engines generally receive a signal from an engine knock sensor and provide a signal for advancing or retarding engine spark timing in response to the signal received from the sensor. This procedure is performed in an attempt to operate the internal combustion engine at optimum efficiency. Some such ignition systems constantly adjust timing in an attempt to maintain, for a given speed, maximum timing advance without knock. A problem with such systems is that the internal combustion engine frequently encounters knock conditions, because the engine is continuously operated proximate the knock threshold.
The prior art also provides an internal combustion engine including an ignition system responsive to a knock sensor, wherein timing is greatly retarded in response to a fault of the knock sensor. See, for example, U.S. Pat. No. 4,621,602. A problem with this design is that, upon fault of the knock sensor, closed loop control of spark timing versus engine speed is completely abandoned in favor of large retardation, thereby resulting in poor fuel economy.
The prior art also provides marine propulsion devices including ignition interruption means for assisting transmission shifting. See, for example, Dretzka et al. U.S. Pat. No. 4,262,622. In these devices engine ignition is generally interrupted when a switch communicating with a shift lever detects movement of the lever, if sufficient shift resistance is encountered. A load sensing device senses shift resistance to movement of a clutch member into or out of engagement with a bevel gear of a transmission, and the engine ignition is interrupted, during shifting, if the shift resistance exceeds a predetermined value. A problem with such a design is that if an operator of the marine propulsion device is slow in actuating the shift lever, the engine ignition may be interrupted for a sufficiently long period that the engine may stall.
The prior art further provides ignition systems designed to skip firing of certain cylinders at certain engine speeds, for the purpose of fuel economy. However, these designs are not particularly addressed to the problem of assisting shifting. A problem may also arise, in skip firing designs, if the same cylinders are continually skipped, thereby creating a gas buildup, which could lead to a backfire condition.